JavaScript : Object-Oriented Programming

Logical operators

There are three operators, called logical operators, that work with Boolean values. These are:

• ! – logical NOT (negation)
• && – logical AND
• || – logical OR

You know that when something is not true, it must be false. Here's how this is expressed using JavaScript and the logical ! operator:

> var b = !true;
> b;
false

If you use the logical NOT twice, you get the original value:

> var b = !!true;
> b;
true

If you use a logical operator on a non-Boolean value, the value is converted to Boolean behind the scenes:

> var b = "one";
> !b;
false

In the preceding case, the string value "one" is converted to a Boolean, true, and then negated. The result of negating true is false. In the next example, there's a double negation, so the result is ...